Intensity modulated radiation therapy (IMRT) is an advanced form of radiation therapy. In IMRT using multileaf collimators, it is critical for the MLC leaves to move accurately according to the pre-designed trajectories to achieve the planned radiation dose distribution with certain accuracy. In contrast to conventional methods, IMRT requires a much more stringent quality assurance (QA) to ensure the normal operation of the delivery system. One of the main quality assurance (QA) tasks is the assurance of positional accuracy of the multileaf collimator leaves. Several studies have indicated that the dose delivery accuracy of IMRT is highly sensitive to multileaf collimator leaf positioning error (See e.g. LoSasso T, Chui C S & Ling C C (1998) Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy, Med. Phys. 25:1919-1927; Budgell G J, Mott J H L, Williams P C & Brown K J (2000), Requirements for leaf positioning accuracy for dynamic multileaf, Phys. Med. Biol. 45:1211-1227). For instance, an error of about 1.0 mm in leaf position could result in more that 10% dose error. Multileaf collimator leaf errors can generally be classified into systematic errors and random errors. A systematic error is referred to as a constant error of all leaves at every leaf position. A random is referred to as an error that may occur at any leaf with an arbitrary value in a certain range. There are two main sources of systematic multileaf collimator leaf position errors, i.e. centerline mechanical offset and imprecise determination of radiation field offset in case of a rounded end MLC (See e.g. Zygmanski P & Kung J H (2001), Method of identifying dynamic multileaf collimator irradiation that is highly sensitive to a systematic MLC calibration error, Med. Phys. 28:2220-2226).
Several methods have been employed to identify and correct the systematic errors; accuracies of better than 3 mm can be obtained (See e.g. LoSasso T, Chui C S & Ling C C (1998), Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy, Med. Phys. 25:1919-1927; Graves M N, Thompson A V, Martel M K, McShan D L & Fraass B A (2001), Calibration and quality assurance for rounded leaf-end systems, Med. Phys. 28:2227-2233; Low W, Sohn J W, Klein E E, Markman J, Mutic S & Dempsey J F (2001), Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy, Med. Phys. 28:752-756). Sources that may affect the magnitude of a random leaf position error include the precision of the multileaf collimator control system, the absolute accuracy of calibration of the multileaf collimator leaf positions and the stability of leaf drive motors. For a multileaf collimator system, a systematic error is relatively easy to handle and once a systematic error is corrected in the system, it would not be necessary to check such an error if the conventional alignment of light fields and radiation fields is performed periodically. On the other hand, a random multileaf collimator leaf positioning error check should be performed on a more regular basis since it is unknown when such an error occurs in a particular leaf.
Currently, in most clinics, the routine QA of MLC is performed using radiographic films with specifically designed MLC leaf sequences as described by Chui et al. (Chui C S, Sprirou S & LoSasso T (1996), Testing of dynamic miltileaf collimation, Med. Phys. 23, 635-641; LoSasso T, Chui C S & Ling C C (2001), Comprehensive quality assurance for the delivery of intensity modulated radiotherapy with a multileaf collimator used in the dynamic mode, Med. Phys. 28:2209-2219). In the method taught by Chui et al., a film is exposed to a dynamically delivered multileaf collimator field that produces a matrix of high intensity regions, for instance about 1 mm wide and 2 cm apart. Subsequently, the film is evaluated for irregularities of the high intensity regions caused by potential leaf positioning inaccuracy. With this method, it is believed possible to visually detect leaf positioning errors as small as about 0.2 mm. However, such a QA performance test is time consuming due to the overhead associated with film irradiation and processing. Furthermore, the film measurement results are difficult to interpret and quantify. Therefore it would, for instance, be difficult to detect errors less than 0.2 mm, which renders the method of film measurement inadequate for QA in IMRT. Accordingly, there is a need to develop new and more accurate methods to improve QA in IMRT.